It has been discovered that the signal-to-clutter ratio for small targets (e.g., targets having a transverse extent less than the antenna beamwidth, and presenting a radial extent less than that represented by the transmitted pulsewidth) may be substantially and significantly enhanced by the pulsed transmission of a plurality of discrete carrier frequencies, representing a plurality of successively increased frequencies, the frequency difference between successive frequencies being not less than that bandwidth corresponding to the recriprocal of the transmitted pulsewidth; and then combining the echoes thereof received from a given target direction and range. A discussion of this technique is more fully discussed in copending U.S. application Ser. No. 430,141 for a Radar System Having Improved Response to Small Targets, filed Feb. 3, 1965, now U.S. Pat. No. 3,500,404, by James O. Anderson et al., assignors to North American Aviation, Inc., assignee of the subject invention, and in copending U.S. application Ser. No. 476,630 for a Multiple Frequency Radar System Having Improved Response to Small Targets, filed Aug. 2, 1965, now U.S. Pat. No. 3,745,578, by Carl R. Barrett, Jr., et al., assignors to North American Aviation, Inc., assignee of the subject invention.
By means of the above-described technique, those components of the received echoes from a small target tend to correlate or cummulatively combine to provide an enhanced signal indicative of the presence of such small target; while the components of the received echoes from a clutter background in the vicinity of the target tend to mutually de-correlate (1) over the duration of the pulsewidth echo for each discrete frequency and (2) as between the received echoes of two discrete frequencies, as to provide an attenuated clutter return, even though the target echo spectra may be contained within the clutter spectra.
While the above-referenced patent applications described several means for implementing such technique, the several embodiments disclosed and illustrated therein employ a noncoherent receiver, whereby the enhancement of the target-to-clutter ratio is less effective. Further, such embodiments do not readily lend themselves to clutter-referenced airborne moving target indication, whereby one discrete target may be identified and distinguished from another by spectral discrimination or differences in doppler shift due to relative velocity differences between them. Such limitation in the noncoherent multiple frequency receiving technique is due to the increased spectral spread in the several spectral components of the received signal, whereby the spectral components for targets of different relative radial velocities (and located in the same general direction and range) tend to overlap in frequency. Accordingly, it is a general object of the subject invention to employ a mutually-coherent multiple-frequency transmission and reception technique for further enhancing the signal-to-noise ratio and target-to-clutter signal ratio in a multiple frequency radar, and which better lends itself to doppler data processing.
In the prior art of doppler radar data processing or moving target indicating (MTI), as applied to a single frequency pulsed radar, several coherent radar techniques have been employed with limited effectiveness, for distinguishing a moving taget relative to a stationary target or a ground clutter return. Such methods have employed means responsive to the relative spectral difference between, or difference in the doppler shifted radar returns for each of, such targets. Where the utilizing radar is mounted on a moving platform such as a high-speed aircraft, the received clutter spectra undergoes a doppler shift in a manner similar to any other target which maintains a relative velocity relative to the radar platform. In other words, both the clutter spectra and moving target spectra will be commonly frequency-translated, or additionally doppler-shifted, as a function of the platform velocity.
In the prior art of airborne moving target indicator (AMTI) systems employing coherent single-frequency radars (in which the receiver comprises a phase detector responsive to the phase of the transmitted single-frequency energy), the frequency of a coherent oscillator (COHO) is shifted in order to compensate for the doppler shift of the clutter spectra (and the target spectra) due to platform (vehicle) velocity and antenna orientation, whereby zero-frequency rejection (i.e., high-pass or doppler) filters may continue to be employed for clutter rejection. Such compensatory frequency shift is accomplished by mixing the output of the COHO with a signal from a tunable oscillator, the frequency of which is controlled by calibrated control source responsive to vehicle speed and antenna orientation. Alternatively, a tunable rejection filter is required to be controlled by the control signal source to reject the doppler-shifted clutter spectra.
In the prior art of noncoherent AMTI systems, advantage is taken of the fact that, in the presence of substantial clutter, the video detected noncoherent receiver signal is clutter referenced, (i.e., the video detected received spectra is folded about a clutter component thereof at zero frequency), regardless of the velocity or changes in motion of the radar platform, whereby a doppler processor having a zero-frequency rejection, or high pass doppler, filter may be employed for clutter rejection. Such prior art single-frequency radar MTI and AMTI Techniques are reviewed more fully in U.S. patent application Ser. No. 391,073 for an AMTI Radar System, filed Aug. 18, 1964, by Forest J. Dynan et al., assignors to North American Aviation, Inc., assignee of the subject invention, and in the text of Chapter 4 of Radar Systems by Skolnik, published by McGraw-Hill (1962).
A disadvantage of the prior art clutter-referenced doppler techniques is that the unipolar video amplitude detection employed provides an output signal which reflects the receiver-gain compression characteristics. Such gain compression, achieved by means of logarithmic receivers or AGC controlled receivers, is conventionally used to overcome the effects of range and the like upon the signal strength of the received signal. However, any compression in the receiver gain characteristics similarly compresses the prior art clutter-referenced doppler video modulation and results in a reduced doppler detection sensitivety.
Recapitulating, a single-frequency noncoherent radar system may directly provide a clutter-referenced video-detected signal for AMTI purposes, while a single-frequency coherent radar system requires the addition of radar-platform velocity-compensation in order to provide a clutter-referenced signal conveniently adapted for AMTI processing. However, such noncoherent technique does not provide the signal-to-noise advantages of coherent data receivers. Also, neither of such single frequency radar techniques provides the enhancement of discrete target-to-clutter ratio provided by multiple frequency radar techniques. Further, such noncoherent clutter-reference signalling technique does not lend itself to use with multiple-frequency radar systems for the reason that the increased spread of each of the resultant beat-frequency clutter and target components of the doppler-shifted spectra tends to result in an overlapping of such spectra as to make difficult the spectral discrimination of a selected moving target from other moving targets or ground clutter proximate thereto and between which a relative radial velocity exists. Moreover, prior art unipolar video amplitude detection of the moving target IF signal results in reduced doppler video output sensitivity due to the gain compression characteristics of the IF amplifier stage preceding the video stage.
By means of the concept of the subject invention, the above-described limitations of the prior art are avoided, and a mutually-coherent multiple frequency radar having a coherent receiver is utilized to provide a substantially clutter-referenced signal inherently adapted for AMTI processing without the necessity of radar platform-motion compensation means, and having an enhanced signal-to-clutter ratio and extended dynamic signal range performance.
In a preferred embodiment of the invention, there is provided multiple radar frequency transmission means responsively coupled to a single modulating frequency source for providing a plurality of concomitant, mutually-coherent transmittted frequencies, uniformly spaced apart in frequency by the amount of such modulating frequency. The source of the modulating frequency, which frequency is preferrably greater than that represented by the reciprocal of the transmitted pulsewidth of the transmitted energy, is also operatively connected as a time coherent, or phase, reference input to a phase detector of a receiver responsive to the beat frequencies between the received echoes of the transmitted energy.
In normal operation of the above-described arrangement, echoes of the transmitted plurality of uniformly frequency-spaced, mutually-coherent frequencies are received and nonlinearly detected, or beat together, to provide a beat frequency spectra. Such beat frequency spectra, when band pass-limited about a center frequency corresponding to the modulating frequency and then phase-detected relative to such modulating frequency, provides a suppressed clutter beat frequency spectrum and an enhanced clutter-target beat frequency spectrum, due to the clutter decorrelation effect manifested by the detected beat-frequency effect among the received echoes of the mutually coherent transmitted frequencies. Further, although the target-to-clutter beat frequency spectrum for each target is displaced in frequency by the amount of the relative doppler shift or difference between the clutter and such target occurring for the transmitted RF frequencies, yet the displacement of the phase-detected clutter spectrum has been discovered not to be similarly shifted by a doppler frequency corresponding to that expected to be associated with the effect of the radar platform velocity upon the transmitted frequencies. Instead, a first component or centroid of the phase-detected clutter spectrum (occurring due to the beat frequencies difference between the component frequencies of the multiple-frequency clutter return and corresponding to the uniform spacing frequency) occurs at a very low frequency approacing d-c or zero frequency, manifesting the effect of the ratio of the spacing frequency to the transmitted frequencies upon the observed doppler shift. Accordingly, such coherently-detected beat frequency signals may be doppler-processed in the manner of clutter-referenced AMTI signals by means of doppler-bandpass or low-frequency rejection means, and without the necessity of the platform motion compensation means associated with prior-art coherent AMTI receiver apparatus. Moreover, because a coherent reference frequency is utilized to phase-detect the IF receiver signals, the resultant video output is therefore less sensitive to the effects of IF receiver gain-compression, the video output being primarily sensitive to the phase difference between the coherent reference and the beat frequencies of the multiple frequency returns.
In other words, the invention provides the advantages of a coherent radar system and of noncoherent AMTI processing, without the normally associated disadvantages of each, by employing a fully-coherent multiple frequency radar system which further provides signal-to-clutter amplitude enhancement, whereby the target signal spectra may be distinguished from the skirt of the clutter spectra at lower target minimum velocities. Accordingly, it is an object of the invention to provide an improved multiple-frequency radar system.
It is another object of the invention to provide a fully coherent multiple frequency radar system.
It is yet another object of the invention to provide a coherent radar system having an enhanced target-to-clutter and signal-to-noise response.
It is still another object of the invention to provide a coherent radar system for a moving platform and providing a substantially clutter-referenced, coherently detected receiver signal without the necessity of radar platform motion compensating means.